Abstract
Abstract. Trace gases play a key role in the chemistry of urban atmospheres. Therefore, knowledge about their spatial distribution is needed to fully characterize air quality in urban areas. Using a new Multi-AXis Differential Optical Absorption Spectroscopy two-dimensional (MAXDOAS-2D) instrument, along with an inversion algorithm (bePRO), we report the first two-dimensional maps of nitrogen dioxide (NO2) and nitrous acid (HONO) concentrations in the city of Madrid, Spain. Measurements were made during 2 months (6 May–5 July 2019), and peak mixing ratios of 12 and 0.7 ppbv (parts per billion by volume) for NO2 and HONO, respectively, were observed in the early morning in the southern part of the downtown area. We found good general agreement between the MAXDOAS-2D mesoscale observations – which provide a typical spatial range of a few kilometers – and the in situ measurements provided by Madrid's air quality monitoring stations. In addition to vertical profiles, we studied the horizontal gradients of NO2 in the surface layer by applying the different horizontal light path lengths in the two spectral regions included in the NO2 spectral analysis: ultraviolet (UV, at 360 nm) and visible (VIS, 477 nm). We also investigate the sensitivity of the instrument to infer vertically distributed information on aerosol extinction coefficients and discuss possible future ways to improve the retrievals. The retrieval of two-dimensional distributions of trace gas concentrations reported here provides valuable spatial information for the study of air quality in the city of Madrid.
Highlights
Air pollution in urban areas has become a concern in our society because it represents a major risk to human health and the environment (WHO, 2019)
Over the past few years, we have reported trace gas measurements in Madrid using the Multi AXis Differential Optical Absorption Spectroscopy (MAXDOAS) technique (Wang et al, 2016; Garcia-Nieto et al, 2018; Benavent et al, 2019), as well as pollutant trend analysis and chemical transport modeling (Borge et al, 2018; Cuevas et al, 2014; Saiz-Lopez et al, 2017)
We provide the retrieval of HONO as an example of the potential of the MAXDOAS-2D to provide spatial information on other trace gases relevant to urban atmospheric chemistry
Summary
Air pollution in urban areas has become a concern in our society because it represents a major risk to human health and the environment (WHO, 2019). In an effort to control and reduce high-pollution events, the local government has enforced some traffic restriction measures (Izquierdo et al, 2020) and has set up several in situ air quality monitoring stations over the city’s metropolitan area. These in situ instruments – as of today – cannot measure some important trace gases present in the atmosphere, and their values are only representative of the immediate surrounding of the instruments and at surface level.
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